A spark plug for use in an internal combustion engine includes, for example, a center electrode extending in the direction of an axis; an insulator provided externally of the outer circumference of the center electrode; a cylindrical metallic shell assembled to the outer circumference of the insulator; and a ground electrode whose proximal end portion is joined to a forward end portion of the metallic shell. The ground electrode is bent at a substantially intermediate portion thereof such that a distal end portion thereof faces a forward end portion of the center electrode, thereby forming a spark discharge gap between the forward end portion of the center electrode and the distal end portion of the ground electrode.
In recent years, in order to improve erosion resistance, there is known a technique for joining a noble metal tip to the portions of the center electrode and/or the ground electrode which are adapted to form the spark discharge gap. According to a proposed method of joining the noble metal tip to an electrode (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-158323), a laser beam is intermittently radiated to the edge of a contact surface between the noble metal tip and the electrode, thereby forming an annular fusion zone composed of a plurality of fusion spots which are strung out, thus joining the noble metal tip to the electrode via the fusion zone. In order to reliably join the noble metal tip to the electrode, a laser beam is radiated in such a manner that fusion spots overlap each other on the surface of the fusion zone. At this time, in order to ensure joining strength for the noble metal tip, there may be formed multi-fusion spots where three or more fusion spots overlap.
The center electrode and the ground electrode may contain aluminum (Al) and silicon (Si) unavoidably, or intentionally for improving oxidation resistance through formation of an oxide film on their surfaces.